The present invention relates to the mounting of electronic modules upon printed circuit boards and, more particularly, to a structure and method for mechanically and electrically connecting the module to the printed circuit board through a plurality of electrical contacts through normal contact force.
In many computer and electronic circuit structures, an electronic module such as a Central Processor Unit (CPU), memory module or ASIC, must be connected to printed circuit board (hereinafter sometimes xe2x80x9cPCBxe2x80x9d). Modules come in a variety of sizes, and common connective dimension lengths are 32 mm, 42.5 mm and 90 mm. In connecting a module to a PCB, a plurality of individual electrical contacts on the base of the module must be connected to a set of a plurality of corresponding individual electrical contacts on the PCB. This set of contacts on the PCB dedicated to receiving the module contacts is known as a land grid array (hereinafter sometimes xe2x80x9cLGAxe2x80x9d) site.
Today""s printed circuit board and associated module circuit densities are so high that distances between contacts within an LGA site as small as 1 millimeter must be supported. In order to connect a module structurally and electrically to an LGA site on a PCB in a reliable fashion, a number of problems must be overcome. Two significant problems that must be addressed are (1) the initial alignment of the respective contacts; and (2) providing a reliable electrical connection between the module and PCB contacts that compensates for mismatches in coefficients of thermal expansion between the module components and the PCB substrate components.
With respect to the first problem, a land grid array site on a circuit board is typically formed by the lamination of a plurality of individual core members interconnected by plated through holes or power vias having upper surface contact areas. These upper surface contact areas are the contacts used to connect the PCB to the module contacts. The individual core members are typically planar epoxy glass dielectric cores having upper and lower surfaces, with a thin planar layer of copper deposed upon the upper and lower surfaces. The copper layers are featurized by etching or other subtractive means to form electrical circuits or power planes. When a copper layer is featurized as a power plane, a plurality of clearance holes or apertures are created through the copper layers and the epoxy substrate, and a plated through hole (PTH) is formed in each aperture to connect the upper and lower surface featurized copper layers and thereby carry an electric signal through the substrate. In laminating the power via apertures, a large amount of substrate material, such as an epoxy resin, is required to fill in all of the vacancies or apertures. Because of the amount of resin that is consumed filling the apertures, the resultant manufactured printed circuit board structure has a slight concave or xe2x80x9cdish-downxe2x80x9d configuration in the area defined by the LGA. Since the upper surfaces of the PCB contacts within the LGA conform to the dish-down shape of the PCB substrate, the LGA contact surfaces do not define a level planar configuration and, therefore, will not align with the planar alignment of a typical module contact array. In a typical PCB, the amount of LGA dish-down is from about 0.0015 inches to about 0.003 inches. What is needed is a way to deform the PCB upward in the LGA area of the board in order to remove this dish-down effect and bring the contacts into a level, planar configuration.
With respect to the second problem, module substrates carrying the connective contacts engaged by the PCB are typically fabricated from ceramic materials. The coefficient of thermal expansion (hereinafter sometimes xe2x80x9cCTExe2x80x9d) of ceramic modules typically ranges from 2 to 10 parts-per-million (ppm). This is much lower than that of a PCB fabricated from an epoxy resin substrate, which will typically have a CTE in the range of about 15 ppm through about 20 ppm. This thermal mismatch results in a shear-strain in the contact connections every time the module/PCB assembly heats up and cools down. The connections located at the corners of the module have the highest amount of shear strain, because they are the farthest from the neutral point at the center of the module; i.e., they have the largest distance to neutral point (DNP) value and, therefore, must withstand the largest displacement force during the heating and cooling cycles of the structure. The strain upon an individual contact connection is quantified by dividing the relative in-plane displacement between the module contact and the PCB contact by the height of the contact connection, also defined as the deformable length of the contact.
A typical prior art means of connecting the module contacts to the PCB contacts is to use solder. The solder is applied in a ball or columnar shape when hot and in a liquid state, and allowed to cool and solidify into a rigid permanent connection. Since the CTE mismatch strain upon an individual contact connection is dependent upon the height of the deformable length of the contact, a typical solder ball, which is about 0.03 inches in diameter, is less preferred than a solder column, which is typically 0.08 inches high. Solder columns accordingly support heating/cooling factor shear strains about three times greater than those supported by solder balls.
However, since solder connections are rigid and cannot move in response to thermal mismatch shear strains, multiple heating and cooling cycles can eventually cause solder connections to develop failures. This is true even in the case of the preferred solder column. Moreover, the application of solder to the contacts is problematic in preventing the solder from spreading outside of the contact areas and causing undesirable shorts between adjacent contacts. And lastly, since solder connections are permanent, they are not desirable for servicing modules by disconnection and replacement in the field. Field technicians do not have a means for disconnecting and reconnecting modules with solder. Therefore, rather than replace an individual defective module, large and more expensive sub-assemblies must be replaced.
Therefore, what is needed is a method and structure for reliably connecting a module to a printed circuit board that will deform the PCB upward and thereby align the PCB and module contacts. What is also needed is a method and structure that provides a reliable electrical connection between the module and PCB contacts that compensates for mismatches in coefficients of thermal expansion between the module components and the PCB substrate components. And lastly, it is also preferred that the method and structure enable quick assembly and disassembly of the module and PCB connection.
The present invention provides a method and structure for connecting a module to a printed circuit board, wherein a substantially rigid interposer having resilient conductors is disposed between a module and a printed circuit board. A clamping means urges the module and printed circuit board toward each other with compressive force upon an interposer positioned therebetween, preferably causing the module and printed circuit board to deform and thereby align their electrical contacts with the surfaces of the interposer. The interposer further comprises a plurality of apertures, each aperture further having a deformable resilient conductor means for connecting a module contact to a PCB contact. The conductor is deformable in shear, which may travel and, therefore, makeup the CTE dimensional mismatch between the module and the PCB. The conductors are detachable, electrically connecting the module and PCB contacts without the requirement of solder or other permanent means.